It has become standard practice to employ pulse-type radars to indicate the presence, or determine the location, of hostile aircraft or other targets under wartime conditions. In some instances, ground-based radars are used to warn of the approach of enemy aircraft, while in other applications radar equipment is designed as an integral part of the so-called "homing" missiles so as to permit them to lock on to and track a target despite any evasive action that the latter may take in an attempt to avoid impact.
Naturally, the widespread use of such radar systems has stimulated activity in the field of countermeasures. This activity has been directed largely to the development of means for effectively "jamming" the radar equipment so as to prevent it from determining the location of a target, this "jamming" procedure usually consisting of the transmission of high-amplitude, high-duty-cycle energy having a frequency as close as possible to the operating frequency of the radar. Such an expedient is not always effective, however, since the reflected pulses from the target in effect "ride upon" the jamming signal and can be detected by the radar through the use of conventional clipping and limiting networks. In order to completely mask the target, it is usually necessary to make the return pulses indistinguishable from the jamming signal radiated by the target insofar as the enemy radar is concerned. It is possible in certain cases to disable the enemy radar by producing a jamming signal of such high power as to saturate the second detector of the radar, or to develop such a large negative AGC voltage that the radar gain is reduced to a point where target reflections are of such low amplitude that they cannot be observed. However, this requires a power source of large capacity, and its size and weight can not always be tolerated on an airborne vehicle.